1. Field of the Invention
This invention relates generally to a TFT display controller.
2. Related Background Art
A new and high-performance TFT technology is coming into its own. This new technology is called Field Sequential Color TFT (FSC-TFT) liquid crystal displays. FSC-TFT displays have larger apertures per pixel. This results in better viewing angles and better transmittance of back light.
For colorization of existing typical TFT displays, a technology using color filters is employed. These displays are called color filter TFT displays. The difference in colorization between color filter TFT display systems and FSC-TFT display systems lies in the way of creating a full range of colors from the three primary colors: Red, Green and Blue. In both types of systems, the luminance (called grayscale levels) of the primary color components lies in a quantized gradient between zero (0) and some upper limit (usually 255). By mixing different gradients of the different primary colors, one can create substantially any desired color. Pink, for example, is a mixture of some value of green combined with an upper limit of red and an upper limit of blue. As green becomes closer to the upper limit, pink becomes closer to white.
In a color filter TFT display, all three color components are generated in close proximity to one another in a small area. The small area is called a pixel, and the three separate components are called sub-pixels. The area is so small that the human eye integrates over the area covered by the three separate sub pixels; and the user does not see three separate primary colors, but instead sees one color that is a combination of the three colors. The pixels are arranged in a two-dimensional matrix called a frame. If each pixel is re-generated every 1/30th of a second, then the display is said to be refreshing at 30 Frames Per Second (FPS). Each pixel and each sub-pixel is refreshing at a rate of 30 Hz. FIG. 1 illustrates one example of a frame of a color filter TFT display system.
In an FSC system, the three color components are generated one at a time in a fast repetitive sequence, all in the same sub-pixel location; and the human eye integrates over time the three separate color components. Each component completely fills and time-shares the pixel area, and there is no concept of sub-pixel areas like the color filter TFT display system. Just like in the color filter TFT display system, the pixels in a FSC system are arranged in a two-dimensional matrix called a frame. And just like in a non-FSC system, if each pixel is re-generated every 1/30th of a second, then the display is said to be refreshing at 30 Frames Per Second (FPS).
However, because the FSC system has no concept of sub-pixel areas, there needs to be another way of identifying the individual components of the pixel. In the FSC system, each color component is associated with a field (i.e., a sub-frame) which is a time division of one frame. Because there are three different color components, there are three different color fields, at least one per each color. Each field corresponds to a sub-pixel of a color filter TFT display system. The red component of all the pixels are refreshed during the red field time, all the pixels are refreshed by the green component during the green field time, and all the pixels are refreshed by the blue component during the blue field time. For a FSC system to refresh the screen at a 30-FPS refresh rate, every field will have 1/90th of a second to refresh. In case, for example, four color fields are assigned to one frame, the frame is refreshed using a red field, a green field, a blue field, and then another green field. This is because the human eye is more sensitive to the color green, and some designs take advantage of this sensitivity to achieve a more crisp display. In such a case, for a 30-FPS refresh rate, each field would have to refresh in 1/120th of a second. FIG. 2 illustrates a 3-field FSC frame and FIG. 3 illustrates a 4-field FSC frame. It can be seen that the same color components of all the pixels (i.e. each field corresponding to each sub-pixel) are displayed at the same time as color fields or color planes.
Keeping the foregoing information in mind regarding frames, pixels, and fields, the concept of sub-fields will now be more easily explained. Just as a frame time can be comprised of three or more field times, a field time can be comprised of a number of sub-field times. Sub-fields can best be understood by first examining TFT Active Matrix display technology with reference to FIG. 4. The Matrix is a grid of columns and lines with one pixel having a transistor at each line and column intersection.
The columns are driven with a source current from devices called source drivers. The source drivers pump measured amounts of voltage corresponding to data to be displayed on pixels into the columns. The lines are driven with a voltage from a device called a gate driver. Each column line will always have some amount of current being pumped into them, but the gate voltage is applied to only one line at a time in the form of a pulse. The pulse on the column output of a gate driver will apply a voltage to gates of all the transistors intersecting that line. Each of these transistors will turn on and allow current to flow from the source driver via the columns to a liquid crystal (LC) capacitor of each pixel. Thus, the LC capacitor will charge. Because each column has an independent measured voltage applied to it in correspondence with data to be displayed on the pixel, each LC capacitor will charge to an independent voltage level for that pixel.
As shown in FIG. 4B, each pixel has a liquid crystal (CLC is the capacitance of the liquid crystal capacitor), a TFT transistor and an auxiliary capacitor CS. The voltage VLC controls the liquid crystal in each pixel area independently to control the amount of light that is allowed to pass through the liquid crystal. The line is connected to the gate of the transistor so that when a gate voltage is applied to the line from the gate driver, the gate of the TFT transistor is gated on. If there is a difference between the voltage VLC applied to the liquid crystal in the pixel of FIG. 4B and the voltage VCOLUMN of the column, where VDS is not 0V, then current will be allowed to flow through the TFT transistor to equalize the voltage VLC to the voltage VCOLUMN of the column. (This current is labeled ID in FIG. 4 with the arrows indicating the direction of current flow). As the current flows, the voltage VLC across the LC capacitor goes up, and the voltage across the TFT transistor goes down. Transmittance of the liquid crystal is determined by VLC. In a normally black liquid crystal, for example, the greater the voltage VLC, the more light is allowed to pass through the liquid crystal. After the gate voltage is removed, the current of the TFT transistor is once again blocked and VLC begins to deteriorate due to leak current or the like. As it deteriorates, more and more light is prevented from passing through the liquid crystal. Eventually no light at all will pass through the crystal and the display screen will be black. In a color filter TFT system there are three sub-pixels for each pixel, and individual sub-pixels are combined with red, green and blue color filters. Therefore, the transistor is gated on once per each frame. The light source is a white light. Looking again at the example of a TFT frame of the color filter TFT display shown in FIG. 1, it can be seen that striped filters covering the entire display would be very effective. In a FSC TFT system where there is no concept of sub-pixels, there are at least three color field times in one frame time, and the transistor is gated on at least once in each one color field.
In view of the above discussion regarding TFT displays, it is apparent that the voltage VLC across the LC capacitor is very crucial. This voltage VLC controls the amount of light that can pass through the liquid crystal, and the amount of light determines how bright the color is. The maximum amount of light possible must be allowed to pass through for each of the three different color components, for example, to obtain a white color. The switching ability of ordinary TFT is not perfect, and cannot hold the voltage across the capacitor constant at the desired level even when the TFT transistor is turned off. FIG. 5 (exaggerated to clearly illustrate the problem) demonstrates how this current flow effects the voltage (VLC) across the LC capacitor over time.
When passing the maximum light to achieve a white color, for example, it can be seen that not long after the gate of the TFT transistor is turned off (i.e. current stops charging the capacitor) the white color will begin to fade to gray and then to black. The ratio between the time period when current is flowing into verses the time period when current is flowing out of the capacitor is high as the drawing suggests. If the display has N lines (or rows of pixels), then the ratio is 1:N. As a result, it would be desirable to alter this waveform.
However, the waveform represents the period of time of a color field. So in order to modify this waveform, the concept of sub-fields must now be introduced. As shown in FIG. 6 (once again, exaggerated to clearly illustrate the problem), if the current is allowed to flow into the capacitor several times during the life time of the color field, the capacitor can be recharged, thus reducing the range of swing in VLC over the lifetime of the color field. Even though color filter TFT systems are not taking advantage of this technique, it could well be applied to them just as easy as it can to FSC-TFT display systems. Because it is the FSC-TFT systems that are presently taking advantage of this concept, the discussions herein below will be directed to FSC-TFT technology with the understanding that every thing covered can easily be adapted to non-FSC TFT technology, i.e. color filter TFT display systems.
It is therefore a main object of the invention to reduce the power consumption of a TFT display apparatus.
A further object of the invention is to enhance the ability of a TFT display apparatus to display moving images.